Phase locked loop (PLL) circuits are well known in the data processing art as clock generators which provide stable clock signals having predetermined, stable frequencies. The stability of each frequency is provided as a result of an iterative process which uses a feedback path to compare an output of the phase lock loop circuit with an input signal typically provided by a crystal oscillator.
Conventional lock detection circuits generally determine lock status by detecting a phase difference between a reference clock signal and the PLL-generated clock (or a divided derivative thereof). This approach generally detects a positive lock status for generated frequencies that are nearly exactly the same as the reference frequency. This characteristic is desirable for many applications. However, for some applications it is desirable to detect lock status within a range of frequencies.
Spread spectrum clock generation is one such application. Electronic devices typically generate electromagnetic interference (EMI) when operating. The EMI generated by one electronic device may adversely affect the operation of another electronic device. In order to minimize adverse effects of EMI on other electronic devices, regulatory agencies in many countries have adopted standards which limit the amount of energy an electronic device may radiate at any given frequency.
Electronic devices frequently use a clock signal of some frequency for operation. In many such devices, long traces or wires are used to route the clock signal to various integrated circuit (IC) components. These long wires or traces can act as antennas which, in turn, radiate energy at the clock signal frequency and (in many cases) its harmonics. Since antennas radiate more efficiently as wavelength becomes smaller with respect to antenna length, the amount of energy so radiated increases as the clock frequency increases. Consequently, in sophisticated electronic devices such as, for instance, personal computers, printers, cellular phones and peripheral devices, where clock frequencies are approaching gigahertz speeds, EMI is increasingly problematic.
When substantially all of a clock's energy is at one frequency, EMI energy may exceed regulatory limits at that clock frequency. A well known technique to reduce the peak EMI energy at the clock frequency (and its harmonics) is to use spread spectrum clock generation techniques to spread the energy across a part of the frequency spectrum. Spectrum spreading is commonly used in radio frequency communication to facilitate high-resolution ranging, multiple access, jamming resistant waveforms, and energy density reduction. Therefore it is desirable to detect the lock status of a spread spectrum clock generator over a range of frequencies rather than at a single frequency.